Timothy Lockwood Armstrong (born November 25 1966) is an American musician best known for his work with punk rock bands Rancid, Operation Ivy, and Transplants, as well as his record label Hellcat Records.

One of the first bands that Tim played in was Basic Radio. After Basic Radio disbanded, he performed in the ska punk band Operation Ivy under the nickname "Lint" from their beginning in 1987, until they disbanded in 1989. Soon thereafter, Armstrong helped found the Dance Hall Crashers, but left the band after a brief period of time. He was also in Downfall with Matt Freeman (Rancid). In 1991 Armstrong went on to greater fame with the very successful Rancid.

While still with Rancid, he also has a side-project, Transplants, which started with Armstrong, his friend "Skinhead Rob" Aston and a drum machine. Blink-182 drummer Travis Barker later came on to replace the drum machine.

In addition to his projects, Armstrong has also been known to pop up on others' albums in various roles. He's contributed vocals, guitar, songwriting to tracks by bands including Box Car Racer, Cypress Hill, Bad Religion, Stubborn All-Stars, Head Automatica, The Specials, The Matches, Left Alone, NOFX, Fruit Cocktail, H2O, Dr. Israel, Agnostic Front, The Lordz, Time Again and Lars Frederiksen and the Bastards, which is a side-project of his Rancid bandmate Lars Frederiksen.

He has collaborated with P!nk, the The Aggrolites, Gwen Stefani, Buju Banton, Kelly Osbourne and Skye Sweetnam.

He has also produced several albums, including the debut albums by AFI, Answer That and Stay Fashionable (1995), The Gadjits, At Ease (1996), F-Minus, S/T (1999) and Lars Frederiksen and the Bastards, S/T (2001).

Armstrong is still very active in the Berkeley scene that grew up around 924 Gilman Street. He has showcased his other artistic talents and created artwork for the Insaints logo. His artwork and photography are also to be seen on various cds of his own label HellCat, eg. Give 'em the Booth compilations, Lars Frederiksen and the Bastards.

A new album by Rancid was set for an early 2007 release, but has been delayed as of recent to the fall of 2008.

He has also done a solo album entitled A Poet's Life, which fans can download for free on the internet. He was releasing one song at a time, and has released a music video with each song.